Zheng Tan (鄭覃) (died 842), formally the Duke of Yingyang (滎陽公), was a Chinese historian and politician during the Tang Dynasty, serving as a chancellor during the reign of Emperor Wenzong. He was viewed as a Li Faction leader in the Niu-Li Factional Struggles.
Background
It is not known when Zheng Tan was born. He came from a prominent line, as his father Zheng Xunyu was a chancellor during the reigns of Emperor Dezong and Emperor Dezong's son Emperor Shunzong. As a result of his father's status, Zheng Tan was able to start his official career as a copyeditor (校書郎, Xiaoshu Lang) at Hongwen Institute (). He subsequently went through the ranks of low-level advisory officials as Shiyi () and then Bujue (); he then served successively as Kaogong Yuanwailang (), a low-level official at the ministry of civil service affairs (吏部, Libu), and then Xingbu Langzhong (), a supervisory official at the ministry of justice (刑部, Xingbu).
During Emperor Xianzong's reign
In 819, during the reign of Emperor Shunzong's son Emperor Xianzong, Zheng Tan was made Jianyi Daifu (), a fairly high-level advisory official. On an occasion when Emperor Xianzong wanted to make five eunuchs directors of troop food supplies for the northwestern border (with Huigu and Tufan), Zheng submitted a petition against the action, and the order was cancelled.
During Emperor Muzong's reign
In 820, Emperor Xianzong died and was succeeded by his son Emperor Muzong. Emperor Muzong was considered pleasure-loving, and in winter 820, there was an occasion when Emperor Muzong met with the imperial officials in the palace. Zheng Tan, his colleague Cui Yan (), and three other advisory officials took the opportunity to advise him that he was spending too much time in feast and games, and was overly exhausting the imperial treasury with his rewards to his favorites. Emperor Muzong was much surprised and initially displeased by the advice, and asked the chancellor Xiao Mian who these officials were. Xiao informed him that these were advisory officials. Emperor Muzong thus understood that they were giving him advice, and he dismissed them with kind words, but did not actually carry out their advice.
Around that time, Wang Chengzong the military governor (Jiedushi) of Chengde Circuit (成德, headquartered in modern Shijiazhuang, Hebei) — whose circuit had long been resistant to imperial rule — died. In light of Wang Chengzong's death, the Chengde soldiers wanted to support his brother Wang Chengyuan as the new military governor, but Wang Chengyuan wished to turn control of the circuit over to the imperial government. Nevertheless, the soldiers were not allowing him to depart and head for his new post as the military governor of Yicheng Circuit (義成, headquartered in modern Anyang, Henan). Wang Chengyuan requested Emperor Muzong to send a senior official to encourage the soldiers to obey the imperial order, so Emperor Muzong sent Zheng to Chengde, assisted by Wang Fan (), to declare the imperial orders and to give a large cash award to the Chengde soldiers. It was said that when Zheng arrived at Chengde, he explained to the soldiers the importance of loyalty to the imperial government, and the soldiers thus allowed Wang Chengyuan to leave.